The invention relates to a method of forming cutouts and transverse welding seams in a web of film that is advanced continuously at high velocity according to the preamble of claim 1.
Such methods are known and are used eg for processing webs of film for plastics bags. In case of a particular type of bags, a perforation line is situated between two welding seams, wherein the one welding seam forms the bottom of one bag, and the other welding seam forms the top of the adjacent bag, and wherein it is possible to separate the thus successive bags in the web by means of the perforation line. In these types of bags, a further central cutout is formed at the top of each bag, whereby separate handle parts remain at each side.
Such a method is disclosed in U.S. Pat. No. 5,573,489. This patent also describes an apparatus for forming plastic bags, wherein a web of film is provided with cutouts and transverse welding seams. A method for forming bags form layers of intermittently advanced film is known from U.S. Pat. No. 3,748,205.
It is necessary to have very low stresses in the welding zone and the subsequent cooling zone since here the welding seam is very soft and will yield even to small stresses. During welding some degree of deformation and changes in the length of the web usually occur, and this has made it necessary to detect each welding in order to be able to position the cutout correctly. This detection is to be carried out for each web of film, and the cutout is positioned relative thereto, also separately for each web, which results in a drastic increase in the costs of the necessary process equipment. Moreover, it becomes difficult to increase the rate of web conveyance since the time available for carrying out the determination of the welding seam position decreases with increased web conveyance rates. The alternative has been to live with certain variations in the position of the cutout relative to the welding.
It is the object of the invention to provide a method by which it is possible to accomplish high web rates and less costly process equipment and whereby it is possible to avoid these known variations while simultaneously eliminating the need for detecting the position of each welding prior to positioning of the cutout.
This is obtained by a method of the kind described in the introductory part of claim 1 and further comprising the method steps mentioned in the characterizing part of claim 1.
When, as stated, the section to be removed from the web is allowed to continue to be attached thereto in a number of support points at the cutting lines that extend transversally of the web, the web can still be exposed to stresses in its longitudinal direction without ensuing deformation, and hereby it becomes possible to freely select at which point in the process the cutouts are to be accomplished. In that case it will be expedient to perform the cutouts using blade parts prior to the weldings and the ensuing deformations of the web of film since the welding can subsequently be carried out completely accurately in relation of the cutouts.
Thereby the welding is the last process to be performed on the web that requires accuracy, and consequently it is not necessary to subsequently determine the exact location of the weldings, as was previously the case. Even in case several adjacently extending webs of film are processed in a die-cutting or cutting unit that extends across all processed webs of film and are operated as one unit, and are subsequently processed in a common welding unit operated in accordance with the same principle, cutouts and weldings will be positioned accurately relative to each other, the cutouts being produced before the weldings.
Obviously the cutting lines described above are not necessarily rectilinear; thus the cutout can be eg circular or oval, and the support points will then be positioned such that stresses in the longitudinal direction of the web can be transmitted from the web to the cutout.
A number of support points can be established in the cutting lines that extend longitudinally in order to ensure that the cutout remains coherent with the web of film in the subsequent process steps.
In particular in the manufacture of bags, it is convenient that a welding seam is formed at both sides of one of the cutting lines that extend transversally to the web of film, and that a perforation is arranged between the welding seams, which extends transversally to the film. Here the perforation serves to enable the individual bags to be separated from each other, and in that case the welding seams will form bottom and top of each their successive bag.
As featured in claim 3, the transverse perforations and one of the cutting lines that extend transversally to the web of film will coincide in an area and be formed in the same process step, and wherein the cutting line constitutes the perforations in this area. The support points of the cutting line have a lower strength than a perforation line, and therefore it will be easier to remove the cutout if it is coherent with the remainder of the web only in the support points of the cutting line and not at the perforation.
According to an alternative embodiment of the invention as featured in claim 4, the transverse perforations and one of the cutting lines that delimit the cutouts transversally extend side-by-side in an area and are formed in each their process step. Here the perforation line will not be torn when the cutout is removed due to the relative strength of this line compared to the somewhat weaker support points in the cutting line.
Removal of the material from the cutouts can be accomplished subsequently in, a simple manner by tearing the support points as featured in claim 5.
According to claim 7 the invention also relates to an apparatus for manufacturing cutouts and transverse welding seams in a web of film that is advanced continuously at high velocity wherein the cutouts constitute sections that are delimited by cutting lines wherein a first workstation is configured for forming cutouts delimited by cutting lines in the longitudinal and transverse directions of the film and that a subsequent welding station is configured for forming transverse welding seams. The apparatus further comprisefist blade parts that form the cutting lines in the longitudinal direction of the web of film, and second blade parts that form the cutting line in the transverse direction of the web of film and have interruptions with a view to forming the second cutting lines with interruptions.
By means of the apparatus it is possible to produce cutouts and welding seams in continuously advanced webs of film, wherein the cutouts and welding seams are accurately positioned relative to each other. This can be accomplished without detections being carried out of the location of welding seams since it is possible with the apparatus to produce the cutouts in the web of film before the weldings.
According to claim 8, the welding station can be configured for forming a welding seam at each their side of one of the cutting lines that extend transversally of the web of film, and such that it is also a perforation tool configured for producing a transverse perforation between the two welding seams.
The features according to claim 8 can be applied when the apparatus is configured for the manufacture of bags.
Convenient embodiments of the apparatus will appear from claims 9 and 10.